“So far, test results obtained from these installations have been encouraging and produced some useful information”
Finished foundation at the building site
imbalance between loads on the different phases of the supply downstream of the break in the neutral. However, if main protective bonding
is provided connecting any extraneous- conductive-parts that exist within the installation – e.g. incoming metal water pipes or gas pipes – to exposed- conductive-parts of the installation, the potential between these will be reduced to a safer touch voltage and danger avoided.
Where exposed-conductive-parts or
extraneous-conductive-parts are located outside – e.g. a metal outside light or a metal pipe connecting a water tap – the main protective bonding will not reduce touch voltages between metalwork connected to the main earth terminal and Earth outside the building. It may therefore be appropriate to provide additional measures such as providing an additional connection to Earth at the consumer’s earth terminal. This is where FEE can be of particular benefit. This may also be an option in situations where extraneous- conductive-parts do not exist within an installation, e.g. due to the use of plastic pipes for incoming water and gas services.
There are a number of options given in Chapter 54 of BS 7671 for earth
electrodes, e.g. earth rods or pipes, earth tapes or wires etc. However, it is often difficult to obtain reliable low resistances to earth when installing these and a FEE is more likely to provide a greater area of influence around the exterior of the building than a single rod electrode. This may be one of the advantages that installing a FEE provides.
Work to date To date, the Institution of Engineering and Technology (IET) JPEL/64 working group research has included a number of
foundation earth electrode installations carried out around the UK, including several in Scotland. The work in Scotland has been kindly facilitated by Ogilvie Homes – part of the Ogilvie Group of companies located in Stirling – with FEE installed at their housing developments in Irvine, Ayrshire, and Bridge of Earn, Perthshire. The FEE installed at these sites comprises of 8mm (CSA of approximately 50mm2
) diameter
galvanised steel bar, installed at a depth of 500mm in the concrete strip foundations, with the work being carried out by DEHN UK Ltd, who manufacture surge protection, lightning protection and earthing products. The subsequent testing carried out
by SELECT made use of a range of test equipment provided by Megger Limited, including their latest DET2/3 high resolution earth tester and MFT 1741 multi-function tester together with an ETK professional earth test kit. So far, test results obtained from these
installations have been encouraging and produced some useful information. These will be further considered and discussed by the JPEL/64 working group as they continue their research into this interesting project. SELECT will continue to provide
Installing 8mm steel conductor
assistance on this research and will keep Members advised on how it may impact electrical installation work in the future as it develops.
CABLEtalk OCTOBER/NOVEMBER 2019 51
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